1. Field of the Invention
This invention relates to valves and more particularly to pinch valves.
2. Background Information
Large mechanical ball and butterfly valves are presently used on railroad tank cars to gate the flow of multi-phase fluids, such as calcium carbonate (limestone) slurry (a common ingredient in many chalky foods and medicines). Since tank cars are invariably exposed to outside temperatures below freezing, it is not uncommon for the slurry to freeze solid in the valve head. This may prevent the valve from opening. Even when opened, the plug of frozen material may be too adhered to clear the valve outlet. In the prior art, special heating jackets are often applied to mechanical valves to melt the slurry around the valve. However, this approach requires valves to be connected to a steam source during unloading of contents, increasing the time, cost and complexity of delivering contents. A valve type that avoids the shortcomings of mechanical ball and butterfly (gated) valves when handling frozen mixtures is highly desirable.
One form of valve that avoids mechanical gates and their disadvantages is the pinch valve. Pinch valves are used desirably for controlling flow of fluids and fluid/solid mixtures in a variety of processes and environments. In general, a pinch valve is a mechanism that provides a variable-diameter pinching force to a flexible or elastomeric tube/hose, which is often termed a “spool.” By pinching, or constricting the spool, the amount of flow therethrough is varied between full, unrestricted flow and shutoff, completely restricted flow. One advantage of pinch valves is that the actuating mechanism is located outside of the tube interior lumen. The fluid never confronts any gates, flaps or other jointed/seamed parts. Rather, the fluid only confronts the pinched (but otherwise unbroken) inner wall of the spool. This greatly reduces the chances of undesirable accretion of particulates in multiphase or colloidal mixtures. In addition, this arrangement allows for isolation of the fluid from any materials that are not suitable for contact with the fluid. For example, many medical and food products can only come into contact stainless steel and food-grade polymers/elastomers. In a conventional, gated valve with crevices, seams or discontinuities invariably lead to contact with non-approved materials as well as the build-up of compounds within the valve mechanism.
However, pinch valves are typically designed for controlling flow in small-diameter tubes and hoses. Their construction tends to dictate a multi-piece, or split, framework that can be disassembled and exposed to allow replacement of the spool and/or assembly and disassembly of the pinch elements and associated gearing. This multi-piece framework is somewhat weak and typically unsealed. Such a design renders the typical pinch valve design unsuitable for heavy industrial applications, like railroad tank cars. Larger pinch valves often employ a screw-driven pinch element that operates on only one side of the spool, thereby causing the spool to spread unevenly during pinching, and compromising durability. Moreover, applicable standards, such as M-1002 Specifications for Tank Cars, promulgated by the American Association of Railroads (AAR), set forth the requirements for strength and survivability in the event of derailment. Conventional pinch valves do not contemplate such standards. For example, skid plates and other structures limit the possible vertical height of a bottom mounted tank car valve. Pinch valves, which often employ external gearing, may require too tall of a frame to provide needed clearance.